1. Field of the Invention
The present invention relates to an apparatus for controlling power transducers of the pulse width modulation (PWM) control type, and more particularly to a control apparatus capable of remarkably improving the waveform of an AC input and/or output voltage or current of the transducers.
2. Description of the Related Art
Power transducers, such as converters converting AC power to DC power and inverters inverting DC power into AC power, are widely used in the various fields of industries. For one of typical examples, there is a power transducer which is constructed by the combination of a converter for converting AC voltage supplied from an AC power source to DC voltage and an inverter for inverting the DC voltage supplied by the converter into AC voltage again and applying the inverted AC voltage to an AC load such as an induction motor.
In this kind of the power transducer, the converter and the inverter are often controlled by the known PWM control. Then, the converter is desired to have the waveform of its AC input voltage, which is as close to a sinusoidal waveform as possible, and in the inverter its output voltage or current is required to be as close to a sinusoidal wave as possible. To meet this, especially the pulse width of the minimum input and/or output voltage or current pulses created by the PWM control should be sufficiently narrow.
In the following, the description will be made of the problem in the PWM control of the power transducer of this kind, taking the case of the inverter as an example. In the typical arrangement of the inverter, as is well known, a main switching circuit, which is composed of six semiconductor switching elements, such as transistors or gate-turn-off (GTO) thyristors, connected as a three phase bridge circuit, is supplied with a DC power through a DC reactor, and the six switching elements are fed with gate signals in accordance with the PWM control to repeat turn-on and turn-off operations in the predetermined manner. As a result, the output voltage or current and the frequency thereof can be controlled.
Thus, in the PWM control system of such inverter generally known in the prior art, a triangular carrier wave and a sinusoidal modulating wave are compared with each other, and gate signals for the switching elements are obtained in accordance with the comparison result.
If an apparatus for such PWM control is realized by the analog circuit components, it is accompanied by a problem of complicated circuit construction requiring a carrier wave generator, a modulating wave generator, a comparator and so on. Further, the apparatus constructed by such analog circuit components still has another problem that the characteristics remarkably fluctuate with the changes in the ambient temperature and the aged deterioration so that it is difficult to obtain the stable operation thereof.
Then there has been proposed an apparatus constructed by the digital circuit components; Japanese Utility Model Laid-open No. 57-3392 published on Jan. 8, 1982, for example. According thereto, both a carrier wave and a modulating wave are generated in the form of digital signals, and gate signals for the switching elements are produced by comparing the digital carrier wave and the digital modulating wave. If the control operation, i.e., the generation of the gate signals as mentioned above, is achieved by a microcomputer thus programed, it is almost occupied by the calculating operation for generation of the various waves and the comparison thereof, so that it can hardly execute other processes.
In this case, moreover, the apparatus mentioned above requires the tremendous amount of storage capacity for storing data for the various carrier waves and the various modulating waves in order to obtain the AC output voltage with the good sinusoidal waveform over the wide range of the frequencies of the AC output voltage. This is not practical because of the construction of data storage means. If the distorted AC output voltage of an inverter is applied to an induction motor as a load of the inverter, for example, another problem is that generation of noise and torque ripple resulting from harmonic components included in the distorted waveform is unavoidable.